Fermi National Accelerator Laboratory FERMILAB-Conf-97/249-E CDF Physics of the Top Quark at CDF Steve Vejcik For the CDF Collaboration University of Michigan Fermi National Accelerator Laboratory P.O. Box 500, Batavia, Illinois 60510 July 1997 Published Proceedings of the 11th Les Rencontres de Physique de la Vallee d Aoste: Results and Perspectives in Particle Physics, La Thuile, Italy, March 2-8, 1997 Operated by Universities Research Association Inc. under Contract No. DE-AC02-76CH03000 with the United States Department of Energy
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CDF/PUB/CDF/PUBLIC/4243 Physics of the Top Quark at CDF Proceedings for Les Rencontres de Physique de la Vallee d'aoste (La Thuile, March 2-8 1997) Steve Vejcik for the CDF Collaboration University of Michigan Abstract Measurements of Top quark properties with the CDF detector are reported. The production cross section and mass provide a consistent picture of the Top quark as described by the Standard Model. Initial studies of other properties such as estimates of branching ratios are also reported.
1 Introduction The Top quark is dened as the weak isospin partner of the b-quark. Its discovery [1] completes the multiplet structure of fermions within the Standard Model and cements the expectations of that theory. Beyond establishing its existence, an experimental program designed to measure its properties can expand the understanding of the Electroweak sector and possibly yield unexpected features. The relevant studies performed at CDF are the measurements of the production cross section ( t t ) and mass (M top ) and the examination of top production and decay features such as branching ratios, the t t invariant mass (Mt t ), and evidence of unexpected or rare decay channels. Additional attention has also been given to understanding what such studies could yield with higher luminosity samples expected in the future. The measurments reported here reect preliminary results from the CDF experiment and are based on recorded luminosities of 109pb 1 obtained at the Fermilab Tevatron. CDF is a general purpose detector designed to detect and measure properties of jets and leptons, missing energy, and identify displaced secondary vertices with a silicon vertex detector. Each of these elements is important for the reconstruction of t t events. The expected decay oft tpairs results in a nal state of a b- and a b-jet and two on-shell W bosons. The subsequent decay of the W bosons classies the decay as either dilepton (both W 's decaying to e or ), lepton+jet (one leptonic W decay and one hadronic decay), or hadronic (both W 's decaying hadronically). The selection of top candidates relies on identiying events passing a series of cuts corresponding to the relevant topology. Dilepton candidates, for instance, are required to contain two isolated lepton candidates, large missing transverse energy ( 6 ET ), and at least two jets. 10 such events are identied as candidate events by CDF with little ( 2 events expected) accompanying background. Additional background rejection is needed in other decaychannels and can be obtained by requiring evidence of b-quark production among the observed jets. Identication of b-jets proceeds either by identifying displaced secondary vertices, expected from long lived b- hadron decays, or by the observation of extra leptons signaling the presence of semileptonic b-hadron decays. The respective eciencies of the two algorithms are approximately 42% and 20%. 2 Measurements of the Production Cross Section The combined branching fraction times eciency for the dierent channels varies from approximately a tenth of a percent to almost 5%. Table 1 shows the measured cross sections for the dierent observed channels. The most precise determination of the cross section is arrived at through the combination of the measurements in the dilepton and
two lepton+jet channels, yielding a measurement of t t =7:5 +1:9 1:7pb. For a top mass of 175 GeV=c 2, the measured cross section is slightly more than one standard deviation from typical predictions of 5pb [2]. Table 1: Measured t t production cross sections from CDF. Channel Acceptance BR(%) Background N obs t t (pb) Dilepton 0:74 0:08 2:1 0:4 9 8:5 +4:4 3:4 Lepton+Jets 3:5 0:7 8:0 1:4 34 6:8 +2:3 1:8 (SVX Tagged) Lepton+Jets 1:7 0:3 24:3 3:5 40 8:0 +4:4 3:6 (SLT Tagged) All Hadronic 4:7 1:6 137:1 11:3 192 10:7 +7:6 -Dilepton 0:119 0:014 1:96 0:35 4 15:6 +18:6 13:2 b-tagged Dilepton 0:51 0:03 1:4 0:3 4 4:6 +4:4 3:1 Combined Lepton+Jets (SVX and SLT) and Dilepton 7:5 +1:9 1:6 Statistical Only 4:0 3 Measurement of the Top Quark Mass The measurement of the Top quark mass is achievable for t t events decaying in dilepton, fully hadronic, or lepton+jet decay channels. The most precise measurement is presently achieved through the complete reconstruction of events in the lepton+jets channel. Such a technique is based on the measurements of jets and leptons in events combined with the kinematic constraints implied by two on-shell W-bosons and a single common mass for the two top quarks in each event. The simultaneous constraint to the kinematics and measurements yields an estimated top quark mass for individual events. The resolution for individual events is dominated by the ambiguity in the assignment of partons to jets and by events with additional jets due to nal and initial state radiation. These two eects increase the intrinsic resolution of 13GeV=c 2 to 30 GeV=c 2. Figure 1 shows the distribution obtained from the data separated into dierent categories based on tagging information. The measured mass is arrived at by comparing the distribution of masses for each tagging category with Monte Carlo models varying by Top mass. The inset plot of Figure 1 shows the simultaneous comparison for all channels to dierent top masses obtained with a maximum likelihood analysis. The minimum of the corresponding parabolic t to this comparison yields the Top mass of 176:84:4 GeV=c 2.
20 18 16 14 CDF Preliminary M = 176.8 ± 4.4 (stat.) ± 4.8 (syst.) GeV/c 2 - log(likelihood) 10 5 Events/ 10 GeV/c 2 12 10 8 0 160 170 180 190 Top Mass (GeV/c 2 ) SVX double tagged 6 4 2 SVX single tagged SLT (no SVX) tagged Not tagged (4 jets with E t 15 GeV) 0 100 150 200 250 300 350 Reconstructed Mass (GeV/c 2 ) Figure 1: Reconstructed mass distributions for candidate t t events decaying through the lepton+jets channel. The inset plot represents the comparison of the distributions with those expected from t t events with dierent masses. The minimum of the subsequent parabolic t indicates the estimated value of M top. The most important systematic uncertainties in the measured mass are due to the uncertainties in the jet E T scale and in the modeling of eects due to hard gluons such as those present in nal state radiation. The respective uncertainties in M top from these eects are 2% and 1:3%. The total systematic uncertainty inm top is estimated to be 2:5%. Future measurements of M top at either the Tevatron or LHC are expected to be dominated by similar systematic eects. If controllable to the level of 1%, a measurement of M top with a 2 GeV=c 2 precision may beachievable in these venues. The measurement ofm top has also been achieved with candidate top events identied in the fully hadronic channel. This measurement relies on a constrained t similar to that used for lepton+jet events and results in M top = 186 10 12 GeV=c 2.
4 Searches for Anomolous Decay or Production Properties The study of candidate t t events allows assumptions about their production and decays to be tested. The conventional decay of one top quark provides a trigger for a relatively unbiassed sample of decays of its partner antiquark. The resulting event sample can then be searched for unexpected or suppressed decay channels which could provide the rst indications of non-standard Model physics. One possibility is the decay t! (;Z 0 )c. Such events would have a signature of a single well identied W -boson, jets, and an accompanying photon or Z 0. The identication of a single candidate, consistent with known background rates, yields 95% condence limits on the respective top branching ratios for the and Z 0 channels of 90% and 2:9%. Other studies include using the reconstructed t t mass spectrum to search for resonant t t production, the non-b-tagged dijet mass spectrum to search for evidence of top decays unaccompanied by aw-boson, and the b-tag multiplicity distribution to measure the branching ratio of B (t! Wb)= (t! Wq). The analysis of both dilepton and lepton+jet events with 4 or more jets provides a lower limit of B>0:58 at 95% condence interval. The result can be recast as a limit on the CKM matrix element V tb > 0:76 at 95% condence limit, assuming unitarity. The picture that emerges from the collective set of studies indicates no evidence of anomolous (non-standard Model) behavior within candidate t t events. 5 Conclusion The discovery of the Top quark represents the achievement of a central goal for the Tevatron collider program and cements the validity of the Standard Model description of the low energy electroweak picture having the multiplet structure expected of a spontaneously broken SU(2)xU(1) gauge theory. The experimental program has subsequently focused on the measurement of production and decay properties. The production rate measured through its cross section ( t t =7:5 +1:9 1:6 pb) is consistent with that predicted by QCD. The full reconstruction of t t events yields a measured value of Mtop = 176:84:44:8 GeV=c 2. No evidence for either anomolous decays or production have been observed. References [1] F. Abe et al., Phys. Rev. Lett.73,225 (1994); F. Abe et al.,phys. Rev. Lett. 74 2626
(1995); S. Abachi et al., Phys. Rev. Lett. 74, 2632 (1995). [2] E. Berger & H. Contopanagos, Phys. Rev. D54, 3085 (1996); S. Catani, M.L. Mangano, P. Nason, & L. Trentadue, Phys. Lett. B378, 329 (1996); E. Laenen, J. Smith, & W.L. Van Neerven, Phys. Lett. B321,254 (1994).